Optical modulator and modulation method

Abstract

An optical modulator comprises an electrode 21 which applies to an optical waveguides 12 of a Mach-Zehnder type optical interference system a first electric signal based on an alternating current signal S 1 and a direct current bias V 1 , an electrode 22 which applies to an optical waveguide 13 of the Mach-Zehnder type optical interference system a second electric signal based on an alternating current signal S 2 and a direct current bias V 2 , and a bias setting section 41 which sets average direct current levels of the first and second electric signals based on signal frequency information D 1 which indicates a magnitude relation between a maximum frequency of the alternating current signal S 1 and a maximum frequency of the alternating current signal S 2.

1. An optical modulator for outputting an optical signal which is modulated based on a plurality of alternating current signals, the optical modulator comprising:
 a Mach-Zehnder type optical interference system, formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide; 
 a first electrode operable to apply, to the first optical waveguide of the Mach-Zehnder type optical interference system, a first electric signal based on a first alternating current signal; 
 a second electrode operable to apply, to the second optical waveguide of the Mach-Zehnder type optical interference system, a second electric signal based on a second alternating current signal having a different frequency from the first alternating current signal; and 
 a bias setting section operable to set average direct current levels of the first and second electric signals based on signal frequency information which indicates a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the second alternating current signal. 
 
     
     
       2. An optical modulator for outputting an optical signal which is modulated based on a plurality of alternating current signals, the optical modulator comprising:
 a Mach-Zehnder type optical interference system, formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide; 
 a first electrode operable to apply, to the first optical waveguide of the Mach-Zehnder type optical interference system, a first electric signal based on a first alternating current signal; 
 a second electrode operable to apply, to the second optical waveguide of the Mach-Zehnder type optical interference system, a second electric signal based on a second alternating current signal having a different frequency from the first alternating current signal; 
 a first direct current electrode operable to apply a direct current voltage to the first optical wave guide; 
 a second direct current electrode operable to apply a direct current voltage to the second optical waveguide; and 
 a bias setting section operable to set, based on signal frequency information which indicates a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the second alternating current signal, an electric potential difference given to the first and second direct current electrodes. 
 
     
     
       3. The optical modulator according to  claim 1 , wherein
 the bias setting section sets the average current direct levels so that a phase of a first optical signal, modulated by the first optical waveguide, proceeds in comparison to a phase of a second optical signal, modulated by the second optical waveguide, when the maximum frequency of the first electric signal is higher than that of the second electric signal, and sets the average current direct levels so that the phase of the first optical signal, modulated by the first optical waveguide, delays in comparison to the phase of the second optical signal, modulated by the second optical waveguide, when the maximum frequency of the first electric signal is lower than that of the second electric signal, and 
 the Mach-Zehnder type optical interference system outputs an output optical signal which is transmitted through a transmission path having a positive dispersion characteristic throughout the entire transmission path. 
 
     
     
       4. The optical modulator according to  claim 1 , wherein
 the bias setting section sets the average current direct levels so that a phase of a first optical signal, modulated by the first optical waveguide, delays in comparison to a phase of a second optical signal, modulated by the second optical waveguide, when the maximum frequency of the first electric signal is higher than that of the second electric signal, and sets the average current direct levels so that the phase of the first optical signal, modulated by the first optical waveguide, proceeds in comparison to the phase of the second optical signal, modulated by the second optical waveguide, when the maximum frequency of the first electric signal is lower than that of the second electric signal, and 
 the Mach-Zehnder type optical interference system outputs an output optical signal which is transmitted through a transmission path having a negative dispersion characteristic throughout the entire transmission path. 
 
     
     
       5. The optical modulator according to  claim 3 , wherein
 the bias setting section sets the average current direct levels so as to satisfy the equation,
   2 nπ<θ 1−θ2<(2 n+ 1)π 
 
 
       when the maximum frequency of the first electric signal is higher than that of the second electric signal, and
 the bias setting section sets the average current direct levels so as to satisfy the equation,
   (2 n− 1)π<θ1−θ2<2 nπ   
 
 
       when the maximum frequency of the first electric signal is lower than that of the second electric signal, where θ 1  denotes the phase of the first optical signal modulated by the first optical waveguide, θ 2  denotes the phase of the second optical signal modulated by the second optical waveguide, and n denotes an integer number. 
     
     
       6. The optical modulator according to  claim 4 , wherein
 the bias setting section sets the average current direct levels so as to satisfy the equation,
   (2 n− 1)π<θ1−θ2< n 2π 
 
 
       when the maximum frequency of the first electric signal is higher than that of the second electric signal, and
 the bias setting section sets the average current direct levels so as to satisfy the equation,
   2 n π<θ1−θ2<(2 n +1)π 
 
 
       when the maximum frequency of the first electric signal is lower than that of the second electric signal, where θ 1  denotes the phase of the first optical signal modulated by the first optical waveguide, θ 2  denotes the phase of the second optical signal modulated by the second optical waveguide, and n denotes an integer number. 
     
     
       7. The optical modulator according to  claim 1 , further comprising:
 a frequency detection section operable to detect frequencies of the first and second alternating current signals, and output values of the detected frequencies as the signal frequency information to the bias setting section. 
 
     
     
       8. An optical modulator for outputting an optical signal which is modulated based on a plurality of alternating current signals, the optical modulator comprising:
 a Mach-Zehnder type optical interference system, formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide; 
 a signal switch section operable to input a first alternating current signal and a second alternating current signal, having a frequency which is different from a frequency of the first alternating current signal, and to switch the elements to which the first and second alternating current signals are output; 
 a first electrode operable to apply a first electric signal to the first optical waveguide of the Mach-Zehnder type optical interference system based on one of the first and second alternating current signals; and 
 a second electrode operable to apply a second electric signal to the second optical waveguide of the Mach-Zehnder type optical interference system based on the other of the first and second alternating current signals. 
 
     
     
       9. The optical modulator according to  claim 8 , wherein the signal switch section switches, based on signal frequency information which indicates a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the second alternating current signal, the elements to which the first and second alternating current signals are outputted. 
     
     
       10. The optical modulator according to  claim 9 , further comprising:
 a frequency detection section operable to detect frequencies of the first and second alternating current signals, and output values of the detected frequencies as the signal frequency information to the signal switch section. 
 
     
     
       11. An optical modulator for outputting an optical signal which is modulated based on a plurality of alternating current signals, the optical modulator comprising:
 a signal combination section operable to generate a first combined signal into which a first alternating current signal and a second alternating current signal, having a frequency different from a frequency of the first alternating current signal, are combined with a first ratio, which is determined by signal frequency information indicating a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the second alternating current signal, and a second combined signal into which the first alternating current signal and the second alternating current signal are combined with a second ratio, which is reverse to the first ratio; 
 a Mach-Zehnder type optical interference systems formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide; 
 a first electrode operable to apply a first electric signal to the first optical waveguide of the Mach-Zehnder type optical interference system based on the first combined signal; and 
 a second electrode operable to apply a second electric signal to the second optical waveguide of the Mach-Zehnder type optical interference system based on the second combined signal. 
 
     
     
       12. The optical modulator according to  claim 11 , further comprising:
 a frequency detection section operable to detect frequencies of the first and second alternating current signals, and output values of the detected frequencies as the signal frequency information to the signal combination section. 
 
     
     
       13. An optical modulation method for outputting an optical signal which is modulated based on a plurality of alternating current signals, the optical modulation method comprising, with respect to a Mach-Zehnder type optical interference system, formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide:
 applying an electric signal, based on a first alternating current signal, to an electrode of the first optical waveguide of the Mach-Zehnder type optical interference system; 
 applying, to an electrode of the second optical waveguide of the Mach-Zehnder type optical interference system, an electric signal based on a second alternating current signal having a frequency which is different from a frequency of the first alternating current signal; and 
 setting, based on signal frequency information indicating a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the second alternating current signal, average direct current levels of the first and second electric signals. 
 
     
     
       14. An optical modulation method for outputting an optical signal based on a plurality of alternating current signals, the optical modulation method comprising, with respect to a Mach-Zehnder type optical interference system, formed on an electro-optic crystal, including a first optical waveguide and a second optical waveguide:
 applying an electric signal based on a first alternating current signal to an electrode of the first optical waveguide of the Mach-Zehnder type optical interference system; 
 applying, to an electrode of the second optical waveguide of the Mach-Zehnder type optical interference system, an electric signal based on a second alternating current signal having a different frequency from the first alternating current signal; and 
 setting, based on signal frequency information indicating a magnitude relation between a maximum frequency of the first alternating current signal and a maximum frequency of the , second alternating current signal, an electric potential difference given to the first and second direct current electrodes.